HF radio wave propagation can be shot nearly straight into the Ionosphere, hence the name Near Vertical Incidence. What goes up, must come down. Knowing that all things have equal and opposite reactions, the angle coming down is also nearly vertical. Then it goes back up, and down again, and over and over. Like this:

Near Vertical Propagation

And it gives you a range Beyond Line of Sight, at the tactical level, like this (minus the TACSAT in the photo):

Tactical Coverage of NVIS Comms

With a regional picture looking something like this (which goes along withPlanning your Footprint) pictured below.

Regional Coverage Using NVIS

So what does this give us? We now have Beyond Line of Sight Communications that do not rely upon repeaters. Once operators are decently trained and have a good amount of time working in this method under their belt, it can be very reliable.

Important to note is that not all HF bands work well for this. Generally speaking, 160-40M work best due to the way the frequencies themselves refract off the ionosphere. Experience as a Radio Operator should tell you when to use which band based on noise level, the amount of heard traffic, and beacon propagation near your operating frequency will give you a good idea if your traffic will be successfully transmitted or not.